This invention relates to a linear response force balance system, and more particularly to an improved mounting of a movable element in a supporting frame of a force balance system.
A preferred embodiment of the invention to be described with reference to the drawings is in a balance of the automatic-null type employing a displacement transducer having a stationary element and a load-responsive movable element. However, it should be recognized that the improved mounting of the movable element to a supporting frame may be used to equal advantage in other force balance systems. Consequently, the present invention is not to be construed as being limited to a balance per se.
In a double-arm beam balance, the weight of a mass is measured by placing the mass on a receiving pan on one arm and balancing the mass with known weights on the other arm until both arms of the beam supported at its center are motionless and level. A single-arm beam balance is similar, but instead of the beam being balanced by added weights at the opposite end of an unknown mass, the pan is preloaded with balancing weights. Once the unknown mass is added to unbalance the beam, balancing weights are removed until balance has been restored. The sum of the weights removed is equal to the unknown mass. This fundamental one-arm type of balance would lend itself readily to automation for those applications which do not require great precision. However, in weighing small masses with 1 mg resolution in a range of 0 to 40 grams, for example, the balance must be carefully leveled on a very stable bench or platform. Otherwise there may be torsion loading due to the difference in the vector of action of the mass-balance system and local gravity. While such torsion loading can be compensated by the use of a calibration mass differential, any additional torsion loading during the weighing process will be uncompensated, such as when the unknown mass is placed on a pan off center, or when known weights are removed from an off center position.
Another problem with a portable balance, besides leveling and calibrating, is achieving the necessary sensitivity for the desired resolution with a construction that is both rugged and economical. What is required for an economical balance that is both sensitive and rugged is some means for suspending a movable element in a supporting frame which has high compliance in a direction normal to the base of the frame and high tolerance for any tilt of the base in providing a linear displacement of the movable element in response to a mass (or force) loaded onto the movable element without the use of any rotational or slide bearings.